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CHAPTER 2LECTURE

SLIDES

The Nature of Molecules andthe Properties of Water

Chapter 2

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Nature of Atoms

• Matter has mass and occupies space

• All matter is composed of atoms

• Understanding the structure of atoms is critical to understanding the nature of biological molecules

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Atomic Structure

• Atoms are composed of– Protons

– Neutrons

– Electrons

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Atomic number

• Number of protons equals number of electrons– Atoms are electrically neutral

• Atomic number = number of protons– Every atom of a particular element has the same

number of protons

• Element– Any substance that cannot be broken down to

any other substance by ordinary chemical means

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Atomic mass

• Mass or weight?– Mass – refers to amount of substance– Weight – refers to force gravity exerts on

substance

• Sum of protons and neutrons is the atom’s atomic mass

• Each proton and neutron has a mass of approximately 1 dalton

Electrons

• Negatively charged particles located in orbitals

• Neutral atoms have same number of electrons and protons

• Ions are charged particles – unbalanced– Cation – more protons than electrons = net

positive charge– Anion – fewer protons than electrons = net

negative charge8

Isotopes

• Atoms of a single element that possess different numbers of neutrons

• Radioactive isotopes are unstable and emit radiation as the nucleus breaks up– Half-life – time it takes for one-half of the

atoms in a sample to decay

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Electron arrangement

• Key to the chemical behavior of an atom lies in the number and arrangement of its electrons in their orbitals

• Bohr model – electrons in discrete orbits• Modern physics defines orbital as area

around a nucleus where an electron is most likely to be found

• No orbital can contain more than two electrons

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Atomic Structure

Energy levels

• Electrons have potential energy related to their position– Electrons farther from nucleus have more energy

• Be careful not to confuse energy levels, which are drawn as rings to indicate an electron’s energy, with orbitals, which have a variety of three dimensional shapes and indicate an electron’s most likely location

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Atomic Structure

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Redox

• During some chemical reactions, electrons can be transferred from one atom to another– Still retain the energy of their position in the atom– Oxidation = loss of an electron– Reduction = gain of an electron

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Elements

• Periodic table displays elements according to valence electrons

• Valence electrons – number of electrons in outermost energy level

• Inert (nonreactive) elements have all eight electrons

• Octet rule – atoms tend to establish completely full outer energy levels

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Periodic Table of the Elements

Principles of Chemistry

• 90 naturally occurring elements• Only 12 elements are found in

living organisms in substantial amounts

• Four elements make up 96.3% of human body weight– Carbon, hydrogen, oxygen, nitrogen

• Organic molecules contain primarily CHON

• Some trace elements are very important

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Chemical Bonds

• Molecules are groups of atoms held together in a stable association

• Compounds are molecules containing more than one type of element

• Atoms are held together in molecules or compounds by chemical bonds

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Ionic bonds

• Formed by the attraction of oppositely charged ions

• Gain or loss of electrons forms ions– Na atom loses an electron to become Na+

– Cl atom gains an electron to become Cl–

– Opposite charges attract so that Na+ and Cl– remain associated as an ionic compound

• Electrical attraction of water molecules can disrupt forces holding ions together

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Covalent bonds

• Form when atoms share 2 or more valence electrons

• Results in no net charge, satisfies octet rule, no unpaired electrons

• Strength of covalent bond depends on the number of shared electrons

• Many biological compounds are composed of more than 2 atoms – may share electrons with 2 or more atoms

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Electronegativity

• Atom’s affinity for electrons

• Differences in electronegativity dictate how electrons are distributed in covalent bonds– Nonpolar covalent bonds = equal sharing of electrons– Polar covalent bonds = unequal sharing of electrons

• O >> N > C H

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Chemical reactions

• Chemical reactions involve the formation or breaking of chemical bonds

• Atoms shift from one molecule to another without any change in number or identity of atoms

• Reactants = original molecules• Products = molecules resulting from reaction

6H2O + 6CO2 → C6H12O6 + 6O2

reactants products

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• Extent of chemical reaction influenced by1. Temperature2. Concentration of reactants and products3. Catalysts

• Many reactions are reversible

Quantifying Molecules

• The molecular weight of a molecule is the sum of the mass numbers of all the atoms in the molecule.

• One mole, or 6.022 1023 molecules, has a mass equal to the molecular weight expressed in grams.

• The concentration of a substance in a solution is typically expressed as molarity (M), which is the number of moles per liter.

6.022 1023 molecules is 1 mole; NaCl’s molecular weight is 58, H2’s molecular weight is 2.

Answer following questions:

How many molecules does 0.5 mole NaCl have?

How many molecules does 0.5 mole H2 have?

How many grams does 0.5 mole NaCl weigh?

How many grams does 0.5 mole H2 weigh?

You need to make 500 ml 1 M NaCl solution, how many gram NaCl do you need?

Example: